The invention relates to a method for transmitting pilot channels in a cellular radio network, in which, in each cell, at least one base station communicates with the mobile stations located within its area. The base stations transmit a data signal in the downlink direction by using transmission directions changing in time and transmit information about the system to the mobile stations on control channels.
In order to operate, mobile stations in a cellular radio network require a great deal of information about the base station in the area in which they are located. In order to be able to communicate with the base station, the mobile stations should, first, be informed of the existence of the base station and synchronized with the transmission of the base station. Also, mobile stations need information, for example, about the network to which the base station belongs and about the traffic channels used. In the present cellular systems, the base stations regularly transmit information of this kind to each cell on channels allocated for that purpose.
The present invention is applicable for use, especially, in a cellular system that uses code division multiple access. Code division multiple access (CDMA) is a multiple access method, which is based on the spread spectrum technique. CDMA has been applied recently in cellular radio systems in addition to the conventional FDMA and TDMA methods. CDMA has several advantages over the conventional FDMA and TDMA methods, for example, spectral efficiency and the simplicity of frequency planning. An example of a known CDMA system is disclosed in the EIA/TIA Interim Standard: Mobile Station-Base Station Compatibility Standard for Dual-Mode Wideband Spread Spectrum Cellular System, TIA/EIA/IS-95, July 1993, EIA/TIA IS-95, which is incorporated herein by reference.
In the CDMA method, the narrow-band data signal of the user is multiplied by a spreading code having a considerably broader band than the data signal. The resulting product has a relatively wide band. In known test systems, bandwidths such as 1.25 MHz, 10 MHz, and 25 MHz have been used. In connection with multiplying, the data signal spreads across the entire band to be used. All users transmit by using the same frequency band simultaneously. A separate spreading code is used for each connection between a base station and a mobile station, and the signals of the different users can be distinguished from one another in the receivers based on the spreading code of each user.
Matched filters provided in the receivers are synchronized with a desired signal, recognized based on the spreading code. The data signal is restored in the receiver to the original band by multiplying it again by the same spreading code that was used during the transmission. Signals multiplied by some other spreading code do not correlate in an ideal case and are not restored to the narrow band. These signals appear as noise with respect to the desired signal. The spreading codes of the system are preferably selected so that they are mutually orthogonal, i.e., they do not correlate with each other.
In a CDMA cellular radio system, it is possible to use a pilot channel in the transmission direction of base station to subscriber equipment, i.e., in the downlink direction. A pilot channel is a data-unmodulated signal which is transmitted with a specific spreading code and uses the same frequency band as the actual traffic channels. The pilot signal is distinguishable from the traffic channels based on the spreading code. The pilot signal is a channel known and listened to by all subscriber equipment within the cell area. The pilot signal, for example, is used in power measurements and in the generation of a coherent phase reference. Each base station of the system transmits its own pilot signal by which the subscriber equipment can distinguish the transmissions of different base stations from each other.
U.S. Pat. No. 5109390, which is incorporated herein by reference, and the aforementioned IS-95 standard discuss a conventional CDMA cellular system which uses a separate pilot channel that is transmitted to the same coverage area with the data signals.
In a typical mobile phone environment, the signals between a base station and a mobile station propagate along several paths between the transmitter and the receiver. This multipath propagation is mainly due to the reflections of the signal from the surrounding surfaces. Signals which have propagated along different paths arrive at the receiver at different times due to their different transmission delays. CDMA differs from the conventional FDMA and TDMA in that the multipath propagation can be exploited in the reception of the signal. The receiver generally utilized in a CDMA system is a multibranch receiver structure where each branch is synchronized with a signal component which has propagated along an individual path. Each branch is an independent receiver element, which composes and demodulates one received signal component. In a conventional CDMA receiver, the signals of the different receiver elements are combined advantageously, either coherently or incoherently, whereby a signal of good quality is achieved.
CDMA systems can also apply a soft handover wherein a mobile station may simultaneously communicate with several base stations by utilizing macrodiversity. The connection quality of the mobile station, thus, remains high during the handover and the user does not notice a break in the connection.
Interference caused by other connections in the desired connection appears in the receiver as noise that is evenly distributed. This is also true when a signal is examined in an angular domain according to the incoming directions of the signals detected in the receivers. The interference caused by the other connections in the desired connection also appears in the receiver as distributed in the angular domain, i.e., the interference is rather evenly distributed into the different incoming directions.
The capacity of the CDMA, which can be measured by means of spectral efficiency, has been further improved with sectorization. A cell is divided into sectors of a desired size that are serviced by directional antennas. The mutual noise level caused by the mobile stations can be reduced significantly in the base station receiver because on average, the interference is evenly distributed between the different incoming directions which are reduced by means of sectorization. The sectorization can naturally be implemented in both transmission directions. The advantage provided in the capacity by the sectorization is proportional to the number of the sectors.
A sectorized cell may also utilize a softer handover wherein a mobile station performs a handover from one sector to another by communicating simultaneously with both sectors. Even though soft handover improves the connection quality and sectorization increases the system capacity, the movement of the mobile stations naturally leads to the stations performing several handovers from one sector to another. This loads the processing capacity of the base station controller. Several soft handovers also produce a situation where several mobile stations communicate simultaneously with more than one (usually two) sector, whereby the increased capacity provided by the sectorization is lost as a signal of a mobile station is audible in a wide sector.
The multiple access interference of the CDMA systems has also been reduced by means of different known multiple access interference cancellation (IC) methods and multi-user detection (MUD). These methods are best suited for reducing the interference produced within the user""s own cell, and the system capacity can be increased to about a double compared to a system implemented without interference cancellation. However, these methods do not significantly improve the size of the coverage area of the base station compared to known technology. Also, the IC/MUD techniques are complicated to realize so they have mainly been developed in the uplink direction.
Another method that has been developed is an SDMA (Space Division Multiple Access) method wherein the users are distinguished from one another based on their location. The beams of the receiver antennas at the base station are adjusted to the desired directions according to the location of the mobile stations. For this purpose, the system uses adaptive antenna groups, i.e., phased antennas, and the processing of the received signal, to track the mobile stations.
The use of the SDMA in connection with the CDMA provides several advantages over the prior methods, such as sectorization. If the sector beams in the sectorization are narrowed in order to increase the spectral efficiency, the number of the handovers to be performed from one sector to another also increases. This in turn increases the calculation capacity required in the base station controller.
In connection with the application of the SDMA, the background art is illustrated in A. F. Naguib, A. Paulraj: Performance of CDMA Cellular Networks With Base-Station Antenna Arrays (Proc. International Zxc3xcrich Seminar on Digital Communications, pp. 87-100, Zxc3xcrich, Switzerland, March 1994), which is incorporated herein by reference. In the SDMA, a signal is received by an antenna group, and the received signal is shaped by digital signal processing so that the directivity patterns of the antennas are suitable for the stages following the shaping in the receiver. In conventional arrangements, the received signal is shaped to maximize the signal-to-interference ratio of the desired signal. The received signal is shaped so that the directivity pattern of the antenna group minimizes the interference caused by the other connections in the desired signal. In the arrangement according to the aforementioned reference, each detected signal component is subject to individual beam shaping, i.e., the impulse response must be known before the shaping.
Experimental Studies of Space-Division-Multiple-Access Schemes for Spectral Efficient Wireless Communications by G. Xu, H. Liu, W. J. Vogel, H. P. Lin, S. S. Jeng and G. W. Torrence (IEEE Int. Conf. On Comm. ICC 1994, New Orleans, USA, IEEE 1994), which is incorporated wherein by reference, discloses a method which applies the SDMA and in which the directivity pattern of the receiver antennas is shaped. However, the method disclosed is suitable for use only in systems where both transmission directions are on the same frequency.
When data signals, i.e., traffic channels, are transmitted to mobile stations according to the SDMA principle by using changing beams, the use of the pilot signal will be problematic in conventional arrangements. In known methods, the pilot signal has been transmitted together with the traffic channels so that the pilot could be used in the mobile stations as a phase reference to enable coherent reception. The pilot signal has also been used in the identification of base stations and as an indicator of a need for a handover. In the case of changing antenna beams, the pilot signal cannot be used as an indicator of a need for a handover.
The purpose of the present invention is to provide a method for transmitting pilot channels in connection with adaptive antenna beams so that pilot channels are used as effectively as possible. The purpose of the arrangement according to the invention is to use the pilot channels both as a phase reference and to facilitate a handover.
A method for transmitting pilot channels in a cellular network is provided. The method includes that in each cell, at least one base station communicates with the mobile stations located within its area. The base stations transmit a data signal in the downlink direction and information about the system to the mobile stations on control channels. The base stations transmit at least one first pilot channel with a predetermined radiation pattern, which determines the cell coverage area, and second pilot in transmission directions that change in time.
The invention also relates to a cellular radio system that includes in each cell, at least one base station communicating with mobile stations located within its area. The base stations transmit information about the system to the mobile stations by using control channels. The system comprises at least one base station, which transmits a data signal to the mobile stations by using radiation patterns that change in time. The cellular radio system comprises at least one base station, which transmits at least one first pilot channel with a predetermined radiation pattern, which determines the cell coverage area, and a second pilot channel in transmission directions that change in time.
In the method according to the invention, adaptive antenna beams are used and thus considerably better spectral efficiency than the conventional cellular systems, including systems applying the CDMA method, is provided. As a result, it is possible to use, for example, the pilot signal both as a phase reference and as a base station detector since both traffic channels and second pilot signals are transmitted by using a common radiation pattern and are subject to the same propagation conditions. Therefore, a pilot signal provides a phase reference for the detection of the desired signal. Correspondingly, a pilot transmitted by using a predetermined unchanging radiation pattern, which may be, for example, an omnidirectional pattern, can be used to detect the need for a handover from one base station to another.
Further, the search according to an invention for advantageous signal components in the angle-of-arrival-time domain is technically advantageous to implement.
According to a first preferred embodiment of the invention, the signal processing can be performed digitally on the base band, whereupon the antenna beams can be oriented directly to the desired directions by the phasing of the received signal. In a second preferred embodiment of the invention, the signal phasing is performed analogically, resulting in a number of fixed antenna beams from which the beams receiving the best components of the desired signal are selected for the reception.